The increasing reliance on communication networks to transmit more complex data, such as voice and video traffic, is causing a very high demand for bandwidth. To resolve this demand for bandwidth, communications networks are relying upon optical fiber to transmit this complex data. Conventional communication architectures that employ coaxial cables are slowly being replaced with communication networks that comprise only fiber optic cables. One advantage that optical fibers have over coaxial cables is that a much greater amount of information can be carried on an optical fiber.
While the FTTH optical network architecture has been a dream of many data service providers because of the aforementioned capacity of optical fibers, implementing the FTTH optical network architecture may encounter some problems associated with legacy systems that are in current use by subscribers. For example, many subscribers of video service providers use set top terminals (STTs), also known as video service terminals (VSTs), to receive and transmit information related to video services. The conventional VSTs are coupled to a coaxial cable. The coaxial cable, in turn, is then connected to fiber optic cables in a hybrid fiber-coax (HFC) system. The coaxial cable from the VSTs in combination with the fiber optic cables can provide a two way communication path between the VST and the data service hub for purposes such as authorizing a subscriber to view certain programs and channels.
For example, conventional VSTs coupled to coaxial cables may provide impulse pay-per-view services. Impulse pay-per-view services typically require two way communications between the VST and the data service provider. Another exemplary service that may require two-way communication passed between the VST and the data service provider is video-on-demand (VOD) services.
For video on demand services, a subscriber can request a program of his choosing to be played at a selected time from a central video file server at the data service hub. The subscriber's VOD program request is transmitted upstream on a return channel that comprises coaxial cables coupled to fiber optic cables. With the VOD service, a subscriber typically expects VCR-like control for these programs which includes the ability to “stop” and “play” the selected program as well as “rewind” and “fast forward” the program.
In conventional HFC systems, a return RF path from the subscriber to the data service hub is provided. The RF return path is needed because a conventional VST usually modulates its video service upstream data onto an analog RF carrier. While the video service upstream data may be modulated onto an RF carrier, it is recognized that the upstream data may be in digital form.
An RF return path typically comprises two-way RF distribution amplifiers with coaxial cables and two-way fiber optic nodes being used to interface with fiber optic cables. A pair of fiber optic strands can be used to carry the radio frequency signals between the data service hub and node in an analog optical format. Each optical cable of the pair of fiber optic strands carries analog RF signals: one carries analog RF signals in the downstream direction (toward the subscriber) while the other fiber optic cable carries analog RF signals in the reverse or upstream direction (from the subscriber). In a more recent embodiment, the upstream spectrum (typically 5-42 MHz in North America) is digitized at the node. The digital signals are transmitted to the data service hub, where they are converted back to the analog RF spectrum of 5-42 MHz. This process typically uses high data rates (at least 1.25 Gb/s) and a fiber or wavelength dedicated to return traffic from one or two nodes.
Unlike HFC systems, conventional FTTH systems typically do not comprise a return RF path from the subscriber to the data service hub because most of the return paths comprise only fiber optic cables that propagate digital data signals as opposed to analog RF signals. In conventional FTTH systems, a downstream RF path is usually provided because it is needed for the delivery of television programs that use conventional RF broadcast signals.
This downstream RF path can support RF modulated analog and digital signals as well as RF modulated control signals for any VSTs that may be used by the subscriber. However, as noted above, conventional FTTH systems do not provide for any capability of supporting a return RF path for RF analog signals generated by the legacy VST.
Another problem with some legacy VSTs and their corresponding video services controller in the data service hub (referred to as the head-end in industry) relates to the timing between downstream RF signals originating from the data service hub and the upstream RF signals generated by a legacy VST in response to the downstream RF signals. One common standard used by the video services industry in which timing between downstream and upstream RF signals is critical is the SCTE 55-2 2002 standard entitled, “Digital Broadband Delivery System: Out of Band Transport Part 2: Mode B.”
The SCTE 55-2 2002 standard is known to those of ordinary skill in the art as a time division multiple access (TDMA) protocol. In a TDMA protocol, each VST is assigned a time interval during which it is to send a message to the data service hub. It may send its message during that time and only during that time interval. The assigned time intervals are controlled by the data service hub, which sends timing assignments to each VST, and which also sends a master time reference, from which each VST measures time in order to locate its message transmission time. Thus, there is a close connection between the downstream time reference and the upstream time at which each VST transmits.
To address TDMA protocols that are used in a FTTH system, one conventional solution creates frequently-recurring time slots referred to as interstitials. One such RF return path solution that uses interstitials is described in commonly owned, U.S. Non-provisional patent application Ser. No. 10/041,299, filed in the name of Farmer et al. on Jan. 8, 2002 and entitled, “METHOD AND SYSTEM FOR PROVIDING A RETURN PATH FOR SIGNALS GENERATED BY LEGACY TERMINALS IN AN OPTICAL NETWORK,” the entire contents of which are hereby incorporated by reference.
These interstitials or interstitial time slots are reserved for any VST that needs to communicate with the data service hub. RF return signals can be processed and transported to the data service hub using these interstitial time slots. At the data service hub, the data in the interstitial time slots can be used to reconstruct the RF return signals into their original analog form. While this conventional solution works as a viable RF return path handling TDMA protocols, the solution can be complex to implement and it can consume a large percentage of available upstream bandwidth. Consuming a large percentage of available upstream bandwidth can reduce the ability of the FTTH system to handle other types of data.
Another problem with some TDMA protocols such as those based on the SCTE 55-2 2002 standard is the handling and processing of control messages. Control messages can include information that is exchanged between a subscriber's VST and a video services controller at a data service hub. The control messages can include information relating to establishing a timing offset for each VST handled by a data service hub. The timing offset can compensate for the distance between a respective VST and the data service hub. Other control messages can include, but are not limited to, power level control messages. Power level control messages can instruct a VST to adjust its RF return signal to a power level such that when the RF return signal is received at the data service hub, it is at sufficient level for processing by the video services controller.
The problem with control messages is that the are designed to work in a conventional HFC plant in which the video services controller expects a response. While it is possible to modify the software in the video service controller of the data service hub to disregard control messaging, it is not always practical and economical to modify the software in the VSTs to disregard this functionality of a TDMA protocol such as those based on the SCTE 55-2 2002 standard.
Another conventional solution for providing an RF return path that has been developed for handling various video service protocols packetizes RF return signals as IP packets and transports them upstream to the data service hub. One such packetizing RF return path is described in a commonly owned, U.S. Non-provisional patent application Ser. No. 10/389,267, filed in the name of Farmer et al. on Mar. 14, 2003 and entitled, “METHOD AND SYSTEM FOR PROVIDING A RETURN PATH FOR SIGNALS GENERATED BY LEGACY TERMINALS IN AN OPTICAL NETWORK,” the entire contents of which are hereby incorporated by reference.
While this direct packetizing of RF return signals can handle some video service protocols, it does not work for TDMA protocols such as those based on the SCTE 55-2 2002 standard. The direct packetizing of RF return signals does not work with the SCTE 55-2 2002 standard because the solution does not maintain the critical timing between the downstream and upstream RF signals. In a packet network design of any type, many packets present themselves for transmission upstream at random times. Usually, packets are handled in the order that they are received. However, some networks have a prioritization system that determines which packets are the most critical and thus should be transmitted first. In any packet network design, packets can be of variable length, so the time it takes to transmit them is variable. This means that the time to transmit other packets will vary, depending on other traffic. The result is an undeterminable time delay in sending packets that causes unacceptable jitter in the packet arrival time of an upstream RF return signal. This is understood by one of ordinary skill in the art.
Another conventional solution to address the problems presented by TDMA legacy VST protocols, such as the SCTE 55-2 2002 standard, uses a device to demodulate the RF return signals immediately at a network subscriber's premises prior to transmission of the signals over the optical architecture. In other words, the RF return signals generated by a VST are immediately demodulated by a conversion device for their information content and this information content is packetized. The packetized (non-RF modulated) return data is sent to a subscriber optical interface for upstream transmission towards the data service hub over the optical architecture. Software in both the VST and the data service hub must be modified. While this approach has the advantage of eliminating the need to handle radio-frequency (RF) modulated signals over the optical architecture, the approach has several major disadvantages. One of the main disadvantages is cost. The conversion device is a very expensive unit (on a per unit basis) that is needed at a network subscriber's premises in addition to a subscriber optical interface that is used to convert electrical energy into optical energy for transmission over the optical network.
Accordingly, there is a need in the art for the system and method for communicating optical signals between a data service provider and a subscriber that eliminates the use of the coaxial cables and the related hardware and software necessary to support the data signals propagating along the coaxial cables. There is also a need in the art for a system and method that provides a return path for RF signals that are generated by legacy video VSTs that may use TDMA protocols, such as protocols based on the SCTE 55-2 2002 standard.
An additional need exists in the art for a method and system for communicating optical signals between a data service provider and a subscriber that can support a TDMA protocol in which timing between upstream and downstream RF modulated signals is important or critical (or both). A further need exists in the art for supporting TDMA protocols that use RF modulated control messages for optimizing information exchanged between VSTs and a video services controller in a data service hub. Another need exists in the art for supporting legacy video service controllers and VSTs with an all optical network architecture.